1. Field of the Invention
The invention relates to an induction furnace suited to operation at temperatures of around 3000° C. and above. It finds particular application in conjunction with the graphitization of pitch fibers and other carbon-containing fibers and will be described with particular reference thereto. It should be appreciated, however, that the furnace is also suited to other high temperature processes, such as halogen purification of graphitic materials to remove metal impurities.
2. Discussion of the Art
Batch induction furnaces have been used for many years for fiber graphitization and other high temperature operations. A typical induction furnace includes an electrically conductive vessel, known as a susceptor. A time-varying electromagnetic field is generated by an alternating current (ac) flowing in an induction heating coil. The magnetic field generated by the coil passes through the susceptor. The magnetic field induces currents in the susceptor, which generate heat. The material to be heated is contained within the susceptor in what is commonly referred to as the “hot zone,” or hottest part of the furnace.
For operations which require high temperatures, of up to about 3000° C., graphite is a preferred material for forming the susceptor, since it is both electrically conductive and able to withstand very high temperatures. There is a tendency, however, for the graphite to sublime, turning to vapor. Sublimation increases markedly as the temperature rises above about 3100° C. Because of variations in temperature throughout the susceptor, furnace life at a nominal operating temperature of about 3100° C. is typically measured in weeks. Life at 3400° C. is often only a matter of hours. Thus, furnaces which are operated at temperatures of over 3000° C. tend to suffer considerable downtime for replacement of components.
Graphitization of carbon-containing fibers, in particular, benefits from treatment temperatures of over 3000° C. For example, in the formation of lithium batteries, uptake of lithium is dependent on the temperature of graphitization, improving as the graphitization temperature increases. Some improvements in the heat distribution throughout the susceptor have been accomplished by measuring the temperature at different points within the furnace during heating using pyrometers. Different densities of induction power are then delivered to multiple sections of the susceptor along its length, according to the measured temperatures. However, pyrometers are prone to failure and need recalibration over time.
To increase the lifetime of the susceptor, it is desirable to cool the furnace rapidly once the high temperature heating operation is complete. Typically, cooling coils carry water around the furnace. However, because the furnace is generally well insulated, it often takes about a week to cool the furnace down from its operating temperature. In some applications, heat exchangers are employed to speed cooling. In such designs, the furnace is cooled to a temperature of about 1500° C. by heat loss through the furnace insulation. Then, valves above and below the hot zone are opened and forced circulation through an external heat exchanger is begun. This system works well for furnaces' that are rarely operated above 2800° C. In furnaces that are routinely operated above 3000° C., the frequent replacement of hot zone components renders these designs expensive to operate. In other designs, the loose insulation material above the furnace is knocked off the furnace to speed cooling. As a result, the insulation needs to be replaced after each furnace run.
The present invention provides a new and improved induction furnace and method of use, which overcome the above-referenced problems, and others.